The technology for sintering polyamide powders under a laser beam is used to manufacture three-dimensional objects, such as prototypes and models. A fine layer of polyamide powder is deposited on a horizontal plate maintained in a chamber heated at a temperature lying between the crystallization point CP and the melting point MP of the polyamide powder. The laser sinters powder particles at different points in the powder layer according to a geometry corresponding to the object, for example using a computer which has the shape of the object in memory and which reconstructs it in the form of slices. The horizontal plate is subsequently lowered by a value corresponding to the thickness of a powder layer (for example, between 0.05 and 2 mm and generally of the order of 0.1 mm), then a fresh powder layer is deposited and the laser sinters powder particles according to a geometry corresponding to this new slice of the object. The procedure is repeated until the complete object has been manufactured. A block of powder is obtained in which the object is present internally. The parts which were not sintered have thus remained in the powder form. Subsequently, the combined product is gently cooled and the object solidifies as soon as its temperature falls below the crystallization point CP. When completely cool, the object is separated from the powder, which can be reused in another operation.
It is recommended for the powder to have a difference MP-CP which is as great as possible in order to avoid phenomena of deformation (or curling) during manufacture. This is because, at the time t0 immediately after the action of the laser beam, the temperature of the sample is greater than the crystallization point (CP) of the powder but the introduction of a fresh, colder, powder layer causes the temperature of the component to rapidly fall below the CP and brings about deformations.
Furthermore, an enthalpy of fusion (ΔHf) which is as high as possible is required in order to obtain good geometrical definition of the components manufactured. This is because, if the enthalpy of fusion is too low, the energy introduced by the laser is sufficient to sinter by thermal conduction the powder particles close to the growing walls but the geometrical precision of the component is no longer satisfactory.
It is clear that everything which has just been explained with regard to the sintering of polyamide powders under a laser beam is valid whatever the radiation which brings about the melting.
For specific uses, it is necessary for the objects obtained to have flame-retardant properties, indeed even fireproofing properties, but also to fulfil criteria for emission of fumes and for toxicity. In the continuation of the text, for simplicity, the term “fireproofing” is used for flame-retardant properties and for fireproofing properties. It is shown that organic phosphorus additives based on an organic phosphinate of a metal and on ammonium polyphosphate are suitable for the laser sintering process. It is sufficient to dry blend these products with the polyamide powder. It has also been discovered that the usual fireproofing agents for polyamides are not all suitable. For example, melamine cyanurate is not suitable.